Chirped Pulse Amplification (CPA) is widely used for generating high-energy femtosecond pulses. This is most
commonly done with a solid-state Ti:Sapphire crystal through a free-space optical path. The high energy density in the
crystal and the precise optical path required with the use of bulk optics make it difficult to design a simple system with
good stability and beam quality over the environmental conditions typically encountered in the manufacturing
environment.
A CPA system using fiber architecture reduces the need for precise beam guiding since the light follows the fiber. The
pump energy is more evenly distributed along the length of the amplifier fiber, reducing the thermal dissipation that is
required (no water chiller is required) and improving the overall efficiency. The fiber architecture also produces a
superior quality beam that does not require great care to maintain.
IMRA's latest FCPA μJewel uses the inherent advantages of the FCPA architecture, along with extensive engineering, to
produce a compact and stable femtosecond fiber laser system. Its high repetition rate and stable performance enables
applications that were difficult to achieve previously.
This paper will review the general design architecture of the FCPA μJewel and discuss several applications.
Fiber lasers offer an excellent technology base for production of an industrial-quality tool for precision microfabrication, answering the need to expand the capabilities of laser material processing beyond traditional welding, cutting, and other industrial processes. IMRA's FCPA μJewelTM femtosecond fiber laser has been developed to address the particular need for direct-write lasers for creation of clean and high-quality micron and sub-micron features in materials of commercial interest. This flexible Yb:fiber chirped-pulse amplification architecture, capable of operating at rep-rates between 100 kHz and 5 MHz, balances the need for higher-repetition rate with that of sufficient pulse energy to work at or near ablation threshold, while meeting industrial standards for temperature, shock and vibration. Demonstration of the need for higher-repetition rates for direct write processes will be provided in this paper. Further, results of laser-processing of materials typically used in flat panel displays, photomasks, and waveguide production using the FCPA μJewelTM laser will be presented.
IMRA's ultrashort pulse fiber laser products continue to evolve to expand the application scope. The latest prototype FCPA produces pulses with less than 500-fs pulse duration at a 50-kHz repetition rate. At the fundamental wavelength of 1045 nm, the pulse energy is greater than 10 mJ. The increase in pulse energy over the standard FCPA μJewel permits greater flexibility in the focusing conditions applicable for micromachining, enabling a wider variety of laser-machined structures and profiles. This paper describes the latest micromachining application areas being studied with this new laser.
High-repetition rate (>200 kHz) ultrafast lasers drive novel heat accumulation processes attractive for rapid writing of low loss optical waveguides in transparent glasses. Heat accumulation is significant at ~1 MHz when thermal diffusion is insufficient to remove the absorbed laser energy from the focal volume (<10-μm diameter) during the interval between pulses. A novel femtosecond fiber laser system (IMRA, FCPA μJewel) providing variable repetition rate between 100 kHz and 5 MHz was applied to waveguide writing in fused silica and various borosilicate glasses in order to investigate the relationship of such heat accumulation effects. Waveguides were formed with <400-fs pulses at 1045-nm at pulse energies of 2.5 μJ at 100 kHz to >150 nJ at 5 MHz. Wide variations in waveguide properties were encountered, particularly when processing 1737F and AF45 borosilicate, at repetition rates greater than 200 kHz. Waveguide characterization revealed unique material-dependent thresholds for cumulative and single pulse phenomenon. Of these materials, fused silica is unusual in resisting waveguide formation at the fundamental wavelength of 1045 nm, but amenable to waveguide writing at the second harmonic of 522 nm. Laser processing windows are presented for several silica-based glasses for creating symmetric waveguides with low insertion loss when coupled to standard optical fibers. The effects of material and laser parameters on thermal accumulation and waveguide characteristics are discussed.
The two-photon absorption (TPA) properties of beta-carotene in chloroform solution were investigated over the 590-790 nm wavelength range. The TPA was characterized using the open-aperture z-scan technique with picosecond pulses provided by a widely traveling-wave optical parametric amplifier (OPA) pumped with a Ti:sapphire laser amplifier. We found the 0.2-1 cm/GW values of two photon absorption coefficient in the 725-790 nm wavelength region. At shorter wavelengths, a dramatic increase in the two-photon absorption around 600 nm was observed. The observed two photon absorption dependence on the excitation wavelength can be explained by the onset of strong excitation from the ground state 1Ag to the higher lying two-photon allowed m2Ag band centered around 310 nm (~4 eV).
Direct waveguide writing with femtosecond lasers can be divided into two general categories based upon the type of lasers used: amplified systems that emit high pulse energy (>2 μJ) at low repetition rates (<250 kHz), and oscillators that produce low energy (<200 nJ) at high repetition rates (>1 MHz). In this presentation, we report on waveguide writing with a novel commercial femtosecond fiber laser system (IMRA, FCPA μJewel) that bridges the gap between these two regimes, providing sub-400 fs pulses with pulse energies of >2.5 μJ at 100 kHz and >150 nJ at 5 MHz. The laser repetition rate can be varied from 100 kHz to 5 MHz in 1 kHz increments through a computer controlled user interface. The ability to quickly and easily vary the repetition rate of this laser was critical in identifying and optimizing laser processing windows for different target glasses. An overview of laser processing windows and waveguide characteristics are presented for borosilicate and fused silica glasses exposed to fundamental (1045 nm) and second harmonic (522 nm) laser light.
Solution-synthesized nanocrystals which manifest strong quantum-confinement effects enable size-tunability of spectral properties and strong excitonic effects. Lead sulfide (PbS) nanocrystals are especially interesting for applications in telecommunication because their resonance is tunable to wavelengths from 1.3-1.55 μm and beyond. In other quantum dot systems, optically-induced bleaching of absorption has been shown to lead to a strong nonlinearity in the vicinity of the exciton peak wavelength [1][2-4][5-8]. We report herein results of picosecond-resolved transient absorption in spin-processible solution-synthesized PbS nanocrystals across the wavelength range 1100 nm to 1600 nm. The sample was synthesized using the solution phase organometallic method (hot injection technique), which provides good control over the size of the nanocrystals [9]. The sample consisted of nanocrystals with diameter around 5.2 nm resulting in an exciton peak at 1330 nm. Since the Bohr radius for the bulk PbS is 18 nm, these nanocrystals lay within the strong quantum-confinement regime [10]. Time-resolved absorption was studied using the single-wavelength collinear pump-probe setup. From the observed double-exponential decay trace of the transmission, fast and slow time constants were extracted. The fast component of few 10is of ps was attributed to Auger recombination. The slow component is on the order of ns. The saturation intensity was also measured in this wavelength range using the Z-Scan technique [11]. The open aperture signals were fit to the intensity-dependent absorption model. The value of the saturation intensity was found to be 0.6 GW/cm2 around the exciton peak.
The crystal growth technique and associated optical characterizations of polydiacetylene PTS , poly bis(p-toluene sulfonate) of 2,4-hexadiyne-l,6-diol, are reviewed in this paper. The commonly observed defects such as twinning and cracking and their origins are analyzed. Two factors were found to significantly decrease the density of defects and improve the quality. First, the monomer crystal was grown reasonably quick consistent with single crystal growth to reduce polymerization. Second, the polymerization rate was reduced dramatically for the 10% to 90% conversion region for a smooth conversion by reducing the polymerization temperature. The absorption spectrum, typical Z-scans for measuring the optical nonlinearity, and SEM pictures of the surfaces are given as measures of the optical quality of the PTS crystals.
A system to measure gaseous hydrocarbon distributions was devised, which is based on IR light absorption by C-H stretch mode of vibration and computed tomography method. It is called IR-CT method in the paper. Affection of laser light power fluctuation was diminished by monitoring source light intensity by the second IR light detector. Calibration test for methane fuel was carried out to convert spatial data of line absorption coefficient into quantitative methane concentration. This system was applied to three flow fields. The first is methane flow with lifted flame which is generated by a gourd-shaped fuel nozzle. Feasibility of the IR-CT method was confirmed through the measurement. The second application is combustion field with diffusion flame. Calibration to determine absorptivity was undertaken, and measured line absorption coefficient was converted spatial fuel concentration using corresponding temperature data. The last case is modeled in cylinder gas flow of internal combustion engine, where gaseous methane was led to the intake valve in steady flow state. The fuel gas flow simulates behavior of gaseous gasoline which is evaporated at intake valve tulip. Computed tomography measurement of inner flow is essentially difficult because of existence of surrounding wall. In this experiment, IR laser beam was led to planed portion by IR light fiber. It is found that fuel convection by airflow takes great part in air-fuel mixture formation and the developed IR-CT system to measure fuel concentration is useful to analyze air-fuel mixture formation process and to develop new combustors.
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